8. What allowance is made for contraction in a wire fence? A railway bridge? Why? 9. Why do the stove-pipes crack when the fire is first started? 10. Why does the house go "thump" on a very cold night? 11. Draw the ball, ring, and spirit-lamp in position. 12. Describe in writing the experiments we have made. QUESTIONS FOR FURTHER INVESTIGATION You have seen that iron and brass both expand. Do they expand equally? Let pupils have a few days to invent a way of answering the question. The experiment may then be tried with the compound bar. See _The Ontario High School Physics_, pages 217-218, also _First Course in Physics_, Milliken and Gale, page 144. If the equipment of the school is limited, it may be necessary to dispense with the ball and ring and generalize from one experiment. Another easily made apparatus consists of two iron rings with handles. One ring will just pass through the other when both are cold. The stove may take the place of the spirit-lamp. A still simpler plan consists in driving two nails into a block at such a distance apart that an iron rod (six-inch nail, poker, bolt, etc.) will just pass between. On heating the rod the increase in length becomes evident. EXPANSION OF LIQUIDS Fill a common bottle with coloured water; insert a rubber stopper through which passes a glass tube about sixteen inches long. Set the bottle in a pan of water and gradually warm the water. The rise of the liquid in the tube will indicate expansion. On setting the bottle in cold water the fall of the column of coloured water shows contraction. See _The Ontario High School Physics_, page 218, also _Science of Common Life_, page 48. Macmillan Co., 60 cents. Set the flask or bottle in a mixture of ice and salt and note that the extreme cold causes contraction for a while, then expansion. Note that when expansion begins, the water has not begun to freeze, but that it does so soon after. The night before this experiment the children should set out in the cold air, tightly corked bottles of water. In the morning they will be found burst by the expansion. APPLICATIONS 1. Why did some of the ink-bottles burst in the cold room? 2. Find large stones split up into two or more fragments. Explain. 3. Why is fall-ploughed land so mellow in spring? 4. Why does ice float? Think what would happen if it did not. 5. Explain the heaving of oats, clover, wheat. 6. Do all liquids expand on freezing? Try melted paraffin. THE THERMOMETER Besides the ordinary thermometer the school should possess a chemical thermometer graduated from 0 deg. Fahrenheit to 212 deg. 1. Our sensations vary so much under different circumstances and in different individuals that they cannot be depended on. Find examples of this and show the need of a measuring instrument. 2. The pupils can learn, by examination of the common wall instrument, the parts of the thermometer--tube, bulb, liquid (alcohol or mercury), and scale.